Various types of optical recording media such as CD-R, CD-RW, DVD-R, DVD-RW, DVD-ROM, MO and so forth are widely recognized and spread as external storages for information processing apparatuses such as computers because they can store a large volume of information and can be randomly accessed easily. With an increase in quantity of handled information, there is a demand to increase the recording density.
Among various optical recording media, optical disks having a recording layer containing an organic dye (also referred to as a dye containing recording layer) such as CD-R, DVD-R, DVD+R and the like are particularly widely used because they are relatively inexpensive and have compatibility with read-only optical disks.
Media such as CD-R representative of optical disks having the dye containing recording layer, for example, are in a laminated structure which has a dye containing recording layer and a reflective layer in order on a transparent disk substrate along with a protective layer for covering the dye containing recording layer and the reflective layer. Recording or reading are performed with a laser beam through the substrate.
In such CD-R, a power calibration area (PCA) for optimization of the recording power of the laser beam (OPC: Optimum Power Control) is set in a portion on the inner peripheral side with respect to the lead-in area, as shown in FIG. 11, for example (refer to Japanese Unexamined Patent Publication No. HEI 9-63061, for example). The PCA is divided into an OPC area and an OPC management area. Each of the areas is comprised of one hundred partitions, and one partition is used in each area for one OPC process. At this time, the partitions in the OPC area are used from the outer peripheral side to the inner peripheral side, whereas the partitions in the OPC management area are used from the inner peripheral side to the outer peripheral side.
In the case of CD-R, when recording on the information recording area is performed with a laser beam, various powers of the laser beam are used to perform trial writing in the OPC area (for example, a partition a1 in FIG. 11), reading of the records written on trials are repeated, the optimum power of the laser beam that can read most appropriately is determined, and the state of use of the OPC area such as the number of times the trial writing has been performed is recorded in the OPC management area (for example, a partition b1 in FIG. 11).
Meanwhile, a recommended recording power value of the laser beam is beforehand recorded in the medium, in general. However, since the optimum power practically varies depending on the medium, it is said to be preferable that the PCA as above is set in each medium to optimize the power of the laser beam each time recording is performed on the medium.
DVD-R (single-sided, single-layer DVD-R), which is representative as well, has a laminated structure in which a dye containing recording layer, a reflective layer and a protective layer covering them are formed in this order on a first transparent disk substrate, and a so-called dummy disk, which is a second disk substrate (which may be transparent or opaque) and a reflective layer formed on the second disk substrate is formed on the protective layer through or not through an adhesive layer. Recording or reading are performed with a laser beam from one side of the disk through the first transparent disk substrate. The dummy disk may be of only a transparent or opaque disk substrate, or may be provided with a layer other than the reflective layer.
Meanwhile, DVD+R has almost the same structure as DVD-R, description of which will be hereinafter represented by DVD-R.
CD-R and DVD-R are optical disks using chemical changes in the dye recording layer, onto which writing is possible only once (that is, rewriting is impossible). On the other hand, CD-RW and DVD-RW are optical disks of the phase-change type using crystalline changes in the recording layer, onto which rewriting can be performed plural times. In such phase-change optical disks, protective layers are formed on and under the recording layer, in general.
In order to largely increase the recording capacity of the optical recording medium, two single-sided DVD-Rs as above are bonded together to form a medium having two recording layers, which is known as a double-sided DVD-R (double-sided, dual-layer DVD-R). Recording or reading are performed by irradiating a laser beam onto each of the recording layers from the both sides (that is, the laser beam is emitted from one side of the medium to perform recording and reading on a recording layer closer to this side, while the laser beam is emitted from the other side of the medium to perform recording or reading on the other recording layer closer to the other side).
Like the CD-R described above, a PCA for the OPC process is set in the known single-sided DVD-R and double-sided DVD-R, as well.
With respect to optical recording media having a plurality of recording layers, there is, in these years, a demand for a single-sided incident type optical recording medium (for example, single-sided incident dual-layer DVD-R) on which recording or reading can be performed on a plurality of recording layers by irradiating a laser beam from one side so as to avoid an increase in size and complexity of the recording/reading apparatus, enable continuous reading from the plural recording layers, and improve the facility.
To meet the above demand, there has been proposed an optical recording medium (DVD-R) as shown in FIG. 12, for example. Namely, there has been proposed a single-sided incident type DVD-R of the dual layer type (single-sided, dual-layered DVD-R) having two recording layers, for example, as the single-sided incident type optical recording medium having the structure below (refer to Japanese Unexamined Patent Publication No. HEI 11-66622).
For example, a single-sided incident type DVD-R of the dual layer type of the laminated type is formed by laminating, on a first light-transmissible substrate 5, a first recording layer 12 made from an organic dye on which information can be optically recorded by irradiating a laser beam for recording, a first reflective layer 13 made of a semi-light-transmissible reflective film that can pass through a part of the laser beam for reading, an intermediate layer 11 that can pass through the laser beam for recording and the laser beam for reading, a second recording layer 12′ made from an organic dye on which information can be optically recorded by irradiating the laser beam for recording, a second reflective layer 13′ reflecting the laser beam for reading, and a second light-transmissible substrate 5′ in this order.
With the above structure, it is possible to record information on both the first recording layer 12 and the second recording layer 12′ from one side of the optical recording medium. In reading, it is possible to read out signals from one side of the medium as being an optical recording medium of the so-called dual-layer type.
In the case of an optical recording medium having two recording layers on which information is recorded by irradiating a laser beam from one side, there is possibility that conditions of the recording or reading vary according to each recording layer because the recording on the second recording layer 12′ is performed through the first recording layer 12, the semi-light-transmissible reflective layer 13, and so forth.
Particularly, the complex refractive index of the first recording layer 12 is changed according to whether or not information is recorded on the first recording layer 12, and thus the quantity of transmitted light is changed. For this, there is possibility that the optimum recording power to the second recording layer 12′ largely varies.
When data is recorded on each recording layer of an optical recording medium (particularly, a single-sided incident type DVD-R of the dual layer type) having a plurality of recording layers, it is necessary to perform the recording at the optimum recording power (optimum power) on each recording layer in order to attain good recording on each recording layer.
When the OPC (Optimum Power control) is performed in an area on the inner peripheral side with respect to the data recording area on each recording layer to obtain the optimum power before the recording is performed on each recording layer, the power of the laser diode (laser power) is controlled to be the optimum power beforehand determined, data is then recorded.
When an electric current is supplied, the laser diode used as the light source of the recording light oscillates a laser power according to the electric current. However, when the laser diode continuously oscillates, the temperature is increased, thus the laser power tends to be decreased even at the same electric current value.
When the temperature rises, the wavelength of the laser beam outputted from the laser diode tends to shift toward the longer wavelength's side. Particularly, CD-R and DVD-R have the maximum absorbed wavelength on the shorter wavelength's side than the wavelength of the laser beam, thus the absorption becomes smaller as the wavelength of the laser beam shifts to the longer wavelength. When the wavelength of the laser beam, which is the recording beam, shifts to the longer wavelength's side, the recording sensitivity deteriorates. Accordingly, a larger laser power is required for stable recording.
Further, the temperature of the laser diode itself changes according to the magnitude of the laser power used for recording, the recording time, the ambient temperature, etc. This causes a change in the laser power.
Even if the electric current value of the laser diode is so set and the laser power is so controlled as to provide the previously-determined optimum power, the laser power actually outputted may change due to a change in temperature of the laser diode, for example, which may make excellent recording difficult.
In the case where data is recorded in one recording layer and data is continuously recorded on the other recording layer, if the data is recorded on the latter recording layer with a laser electric current value corresponding to the optimum power beforehand determined, there is possibility that the data is not recorded or the recording is insufficient because the laser power is insufficient, which brings failure in excellent recording.
Particularly, when data is continuously recorded on plural recording layers, it is impossible to perform the OPC on each of the recording layers immediately before the recording is performed on each of the recording layer. For this reason, there is no other alternative but to use the optimum power determined in the OPC beforehand performed. It is thus impossible to cope with changes in temperature of the laser light source, which obstacles attainment of excellent recording in each recording layer.